A process in which a gas mixture is separated adsorptively into two components is disclosed in German patent document 24 41 447. In this system, the rectification of air into an oxygen-enriched product fraction and a product fraction consisting substantially completely of nitrogen is disclosed.
The adsorbent is a molecular sieve coke having a high capacity for oxygen.
During the adsorption phase the gas is passed through an adsorption bed charged with the molecular sieve and a product fraction is withdrawn which is enriched to greater than 99 vol. % with nitrogen. The adsorption is carried out at superatmospheric pressure and the desorption, yielding a gas fraction enriched to about 35 vol. % with oxygen, is effected by evacuation.
The process is carried out in at least two adsorbers with cyclic functional interchange thereof.
An important feature of this process is that only one of the components in the original gas mixture, namely air, is highly enriched in the corresponding product fraction. In this case, the highly enriched product fraction is that which contains nitrogen.
Similar processes have been proposed for other gas mixtures. For example, it is known to effect selective adsorption of individual components of various hydrocarbon mixtures with zeolitic molecular sieves. These processes utilize the varying selectivity of different molecular sieve types for various hydrocarbons.
For example, for the recovery of the lightest n-paraffins, namely, methane and ethane, the molecular sieve which is used is of the type 4A. For C.sub.3 and for higher n-paraffins, molecular sieves of type 5A are used. For the selective recovery of C.sub.4 and higher i-paraffins, molecular sieves of types 10X and 13X are selectively used because they preferentially retard the passage of these hydrocarbons.
With the aforementioned molecular sieve types for the rectification of hydrocarbon mixtures which contain more than two valuable components or components which are to be recovered, it has been possible heretofore in a single adsorption step only to recover one component in highly concentrated form while the other component, at best, is obtained with a significantly lower enrichment.
It is thus necessary to provide additional adsorptive separation stages to recover highly concentrated fractions for these other components.
Since a number of successive adsorption stages must be operated in cyclical functional interchanging relationship, the apparatus with its cascade of stages is comparatively expensive and difficult to operate and to maintain.